Why do kids get fussy & sentimental when your expecting?

firt time mommy here

I am 5 weeks pregnant and I have a 3 yr old step daughter that currently is only with us weekends. So I told my fiance that we will not mention anything to our little one until i have a big belly. Well two days ago she facetimed everyone in her contact list from her moms iPad crying saying she wants me and wants me to pick her up. So when my fiance called the mom she asked him if we we're pregnant? Of course he told her no but she said that's the only explanation she has to why shes been crying for me. Than my MIL called him telling him the same thing.

Anyone know why this happens?